1. Field of the Invention
The present invention relates to a discharge lamp device that lights a discharge tube, and more particularly to a discharge lamp device that lights a discharge tube filled with a rare gas and provided with a first electrode at the inside thereof and a second electrode at the outside thereof. The present invention relates further to a backlight provided with such a discharge lamp-device.
2. Description of the Related Arts
There has been proposed, for example, a rare gas discharge lamp device disclosed in Japanese Laid-Open Patent publication No. 6-163005, as a discharge lamp device that applies a voltage to a discharge tube filled with a rare gas and having an internal electrode mounted at its inside and an external electrode mounted at its outer peripheral surface from a driving circuit connected to the internal electrode and the external electrode for driving the discharge tube.
Regarding lighting control to the discharge tube which has the internal and external electrodes, there are documents such as Japanese Laid Open publication Nos. 2002-75682 and 2001-267093, in which an alternating rectangular wave voltage is applied as a driving voltage.
FIG. 14 is a view showing a configuration of a conventional discharge lamp device. In this figure, a discharge lamp device 10L has a cylindrical discharge tube 103 filled with a rare gas. The discharge tube 103 has an internal electrode 101 mounted at the inside of the discharge tube 103, and a band-shaped external electrode 102 mounted along the tube axis direction of the discharge tube 103 at its outer peripheral surface.
The internal and external electrodes 101 and 102 are respectively connected to a driving circuit 105 which causes the discharge tube 103 to emit by applying an alternating rectangular wave voltage.
FIGS. 15A to 15C are waveform charts showing waveforms of an applied voltage or the like in the conventional discharge lamp device. FIG. 15A shows a waveform of an alternating rectangular-wave voltage applied between the internal and external electrodes 101 and 102, in which the axis of ordinate represents an applied voltage (V) between the internal and external electrodes 101 and 102 when the potential of the external electrode 102 is defined as a reference potential, while the axis of abscissa represents a time (s), respectively.
FIG. 15B shows a waveform of electric current flowing through the internal and external electrodes 101 and 102, in which the axis of ordinate represents current (A), while the axis of abscissa represents a time (s), respectively. Further, FIG. 15C shows a waveform of a brightness, in which the axis of ordinate represents a brightness (cd/m2), while the axis of abscissa represents a time (s), respectively.
In case where the alternating rectangular-wave voltage that alternately becomes a positive voltage and a negative voltage is applied as shown in FIG. 15A, positive current (current at the rising edge) having a differential wave shape corresponding to a rising edge of the positive voltage and negative current (current at the falling edge) having the differential wave shape corresponding to a falling edge of the negative voltage are alternately flown as shown in FIG. 15B.
A phosphor in the discharge tube 103 is excited when the positive current or negative current flows therethrough, to thereby provide a brightness characteristic shown in FIG. 15C. Specifically, the brightness is shown as a waveform L1 corresponding to the positive current and a waveform L2 with a brightness smaller than that of the brightness waveform L1 corresponding to the negative current.
As shown in FIG. 15C, the brightness characteristic (brightness waveform L2) upon applying the negative voltage has a brightness lower than that of the brightness characteristic (brightness waveform L1) upon applying the positive voltage by a level shown by a reduced-brightness waveform L4, thus to reduce a luminous efficiency. This is considered that a contraction discharge occurs upon applying the negative voltage, causing a brightness reducing action due to this contraction discharge, thus to lower the brightness.
Hence, the conventional method for applying an alternating rectangular-wave voltage brings a reduction in brightness every half cycle of the alternating rectangular-wave voltage, so that a sufficient luminous efficiency cannot be obtained.